Is the circadian core temperature rhythm of juvenile rats due to a periodic blockade of thermoregulatory thermogenesis?

Previous studies have demonstrated an endogenous circadian rhythm of core temperature (Tc) in suckling-age rat pups. Our aim was to establish whether the low and irregular Tc at the beginning of the light phase is caused by a temporary blockade of thermoregulatory thermogenesis. We therefore isolated and artificially fed 10-day-old pups for 30 h at five ambient temperatures (Ta), ranging from thermoneutrality to severe cold loading, while continuously recording Tc and metabolic rate (MR). During the maximum phase of the Tc rhythm MR increased linearly with decreasing Ta--to as much as 260% of the daily mean MR at thermoneutrality (TNMR)--so that Tc decreased less than 1 degree C with increasing cold load. During the minimum phase, the MR at all but the lowest Ta fell to, or even below, the TNMR and the amplitude of the Tc rhythm increased from 2 to 5 degrees C with increasing cold load. Under the most severe cold load, however, a further decrease of the minimum Tc was prevented by an increase of MR to 135% of the TNMR. Supplementing the continuously fed synthetic milk with noradrenaline (1.2 mg kg-1 h-1) during the minimum phase of the Tc rhythm increased MR upto 290% of the TNMR. The loose regulation of Tc during the minimum phase of the juvenile circadian Tc rhythm is thus not caused by a peripheral impairment of thermogenic capacity at the beginning of the light phase.